A fiber-optic amplifier with the features recited there is known from M. Shimizu et al, "Compact and Highly Efficient Fiber Amplifier Modules Pumped by a 0.98-.mu.m Laser Diode", Journal of Lightwave Technology, Vol. 9, No. 2, February 1991, pages 291 to 295. In those fiber amplifier modules, the active laser medium contained in the light-signal-amplifying section of fiber is erbium, Er.sup.3+ ions to be exact. The pump laser is a semiconductor laser, also called "laser diode". It emits light at a wavelength of 980 nm. This light, called "pump light", is fed into the Er.sup.3+ -doped fiber section through a fiber-optic coupler. The pump laser is driven by a direct current as an operating current.
Such a drive of the pump laser involves the following problem: The semiconductor laser driven by the direct current is generally single-mode, i.e., it emits light at a single wavelength, e.g., 980 nm, or it emits in only few modes, i.e., it emits light with two or more wavelengths, e.g., with three wavelengths of 978 nm, 980 nm, and 982 nm. In such modes of operation, a semiconductor laser is sensitive to back reflections of the pump light. Such back reflections of the pump light generated by the pump laser can occur at all points in the propagation path where refractive-index step differences are present, e.g., in the area of the fiber-optic coupler, at the ends of the erbium-doped fiber section or at a succeeding isolator. This changes the efficiency of the semiconductor laser; the pump light reflected back may cause variations in the intensity of the pump light emitted by the semiconductor laser since the active layer of the semiconductor laser, together with a part of the transmission link (i.e., up to the point of the refractive-index step difference), acts as a laser resonator. This results in a change in the wavelength of the pump light emitted by the pump laser and, on the other hand, causes the intensity of the pump light to oscillate. Particularly if the oscillations occur in the low-frequency range, e.g., in the hertz or kilohertz range, the amplifying effect of the fiber section will be temporarily neutralized: under certain circumstances, the optical transmission may even be interrupted.
If two pump lasers (both semiconductor lasers) are present (known from K. Nakagawa et al, "Trunk and Distribution Network Application of Erbium-Doped Fiber Amplifier", Journal of Lightwave Technology, Vol. 9, No. 2, February 1991, pages 198 to 208, particularly FIG. 3c on p. 199), the problem described exists for each of the two pump lasers. Another problem in that case is that residual pump light from each of the pump lasers reaches the respective other, opposite pump laser and causes optical instabilities, which, in turn, lead to the above-described gain-variation problems. Therefore, optical isolators are generally used in front of the pump lasers.
As a solution to the problem just described, the prior European Patent Application EP 92906005 proposes to modulate the direct current commonly used as operating current by an alternating current, with the frequency of the alternating current lying above a frequency determined by the reciprocal lifetime of that energy level of the active laser medium whose decay causes the amplification of the light signals.